Some Appointments are more important than others.
I would find it helpful, if there would be - for example - 3 or 5 importance levels to choose for appointments to set while typing in a new appointment. These levels should be shown in the popup alarm window as well, so that the most important appointments can be seen immediately.
Maybe the levels could be shown in colors from green over yellow to red (when 3 levels are active - or e.g. grey - green - yellow - orange - red if 5 levels are given). The standard level should always be the lowest (3 levels) or second lowest level (5 levels) to let new appointments been down- or upgraded.
Despite the fact that different calendars can have different colors themselves, the importence levels could be displayed in colors in a little box or as a colored line on the left side of the appointment in the calender overview.